The purpose of my proposed research project is to investigate the mechanism of regulation of proximal tubular acidification. Specifically, I will investigate the effects of 1) peritubular pH or bicarbonate concentration, and 2) NaCl and water absorption, on HCO3 absorption by in vivo continuous microperfusion of the rat proximal tubule. Preliminary results have demonstrated that proximal acidification consists of two components: 1) transcellular proton secretion which demonstrates saturation kinetics when examined as a function of luminal bicarbonate concentration, and 2) a small passive bicarbonate leak. Increases in peritubular pH inhibited proton secretion and decreased the maximal rate. Decreases in peritubular pH had the opposite effect. In the firts part of my project, I propose further investigate the effect of peritubular pH with respect to: 1) the time course of the pH effect (peritubular pH will be changed with peritubular capillary perfusion (duration minutes), intravenous infusions (duration hours), and chronic feeding (duration days); 2) whether the peritubular effect is due to pH or [HCO-3]; 3) the role of membrane insertion of preformed transporters; 4) the role of chronic adaptation in the absence of altered peritubular pH at the time of study; and 5) the possible role of hormones in mediating the chronic adaptation. In related studies I will examine the effect of glucocorticoids on proximal acidification. In collaboration I will examine the effect of in vivo acid-base abnormalities on Na/H antiporter activity assayed in vitro utilizing a brush border membrane preparation. In order preliminary studies, I have found an effect of flow on proximal acidification which I attributed to the presence of a luminal diffusion barrier. In the second part of my project, I will further investigate the possibililty of a luminal diffusion barrier and the possibility of a diffusion barrier on the peritubular side of the cell. I will examine the effect of NaCl and H20 flux on the rate of bicarbonate absorption. If diffusion barriers exist and water moves through them, bicarbonate should be dragged and the rate of proton secretion stimulated. Kinetic analysis will be used to differentiate a luminal or peritubular convection effect. The proposed studies along with my previous results will help develop a thorough understanding of the kinetics and regulation of proximal acidification.